Retroviruses enter cells following binding to specific cellular receptors and can be grouped according to which receptor they use. Members of a given group presumably use the same receptor as they interfere with infection by other members of the same group. Members of different groups do not so interfere and enter cells by interaction with different receptors. Not only the role of receptors in viral entry remains poorly understood, but also their function in cell differentiation, development or physiology remain obscure, functions they are likely to have since their evolution solely to allow viral entry seems improbable. Tissue-specific expression of receptors may also play a role in malignant transformation by directing retroviral entry specifically into transformation susceptible cells. We propose to clone and study human genes encoding receptors for two different retroviruses. This proposal is based on the recent development of retroviral vectors carrying genes which can be selected for when in the proviral state. By transfecting mouse cells with human DNA and then infecting the mouse cells with appropriate pseudotypes of a selectable virus, we have identified mouse cells carrying two different human receptor genes and have extended this to secondary transfectants for one of the genes. We propose to; (a) clone human repetitive DNA sequences from a secondary transfectant for the Gibbon Ape Leukemia Virus (GALV) receptor, (b) identify clones that have single copy human DNA sequences present in each of 5 independent primary GALV-receptor transfectants. Such sequences should be part of or close to the GALV-receptor gene, (c) use the single copy probes and chromosome-walking to isolate most of the gene from a representative DNA library constructed from a secondary transfectant. This will not only provide sequences to be used below but will also define the size and general boundaries of the gene, (d) identify which of the cloned DNA sequences are represented in RNA transcripts and then use these to obtain cDNA clones, (e) determine the nucleotide sequence of the cDNA clones and from that a deduced amino acid sequence of the protein, (f) examine the tissue expression patterns of the RNA transcript(s), (g) determine if the GALV receptor is related to the Cat Crandell Cell (CCC) virus human receptor gene by Southern analysis of CCC-receptor transfectants with GALV receptor probes. The CCC gene would then be cloned by using its homology with the GALV-receptor gene, or by the route described for the cloning of the GALV-receptor gene.